The present invention relates generally to micro- and nano-scale patterned, metallic structures and methods of making such structures. More particularly, the present invention relates to metallic structures comprising precise, three-dimensional structures replicated from a patterned template substrate and methods of making such structures.
Several methods are known for fabricating patterned metal surfaces with features on a sub-micrometer or nanometer length scale. For example, in one method a metal film is deposited on a surface of a substrate such as by using thermal evaporation or sputtering. After depositions the metal film is patterned to have sub-wavelength scale features by conventional lithography steps such as by using photolithography or e-beam lithography. Alternatively, after deposition, focused ion beam (FIB) milling is used to pattern the metal film. Using either approach, sub-micrometer features can be formed in the metal film. These features, however, have several shortcomings. For example, one limitation is the surface roughness. Another limitation is the low throughput of lithography steps such as e-beam or FIB milling.
Additional known methods for fabricating patterned metal surfaces include nanoimprinting and nanomolding. Although nanoimprinting and nanomolding can pattern metals on the proper length scales, again, undesirable surface roughness is usually present in metal surfaces formed by nanoimprinting and nanomolding. In a typical proem, a patterned polymeric mold is filled with metal to form a replica. This produces undesirable surface roughness because metals do not easily wet the surfaces of the polymeric mold. Moreover, an additional shortcoming of nanoimprinting and nanomolding is that the polymeric mold needs to be etched away from the metal film to release the metal film. Accordingly, each mold can only be used once to produce a single metal film.
Another technique that can be used to fabricate smooth metal surfaces is generally referred to as template stripping. Template stripping utilizes the poor adhesion and good wettability of noble metals on solids such as mica, glass, and silicon. In a typical template stripping process, a freshly cleaved mica surface is coated with a film of gold. The exposed surface of the metal is then attached to another substrate with an epoxy adhesive. When the mica and substrate are separated the gold adheres to the substrate by the epoxy and is released by the mica surface. Such a method, however, is limited to use with generally flat surfaces and has not successfully been utilized with surfaces including three-dimensional features such as those typically found on patterned metal films. This is because the addition of three-dimensional features generally increases the area of mica in contact with gold. As this contact area increases it becomes more difficult to separate the gold film from the mica surface. Moreover, such three-dimensional features can interfere with separation of the gold from the three-dimensional surface features. Where a patterned metal having three-dimensional features is desired, the above nanoimprinting and nanomolding techniques are typically used wherein the mold is etched away from the metal film.
Yet another technique provides methods for replicating patterned metal films from a template substrate, the metal films being suitable for use in plasmonic devices and metamaterials. The template substrate is reusable and can provide plural copies of the structure of the template substrate. Moreover, because high-quality substrates that are inherently smooth and flat are available, patterned metal films can provide surfaces that replicate the surface characteristics of the template substrate both in the patterned regions and in the unpatterned regions. See, for example, PCT application WO 2010/065071 to the Regents of the University of Minnesota.